Accompanying the increasingly higher levels of integration of semiconductor devices in the semiconductor industry in recent years, there is a need for fine patterns that exceed the transfer limitations of conventional photolithography methods using ultraviolet light. Extreme ultraviolet (EUV) lithography is considered to be a promising exposure technology that uses EUV light to enable the formation of such fine patterns. Here, EUV light refers to light in the wavelength band of the soft X-ray region or vacuum ultraviolet region, and more specifically, light having a wavelength of about 0.2 nm to 100 nm. Reflective masks have been proposed as transfer masks for use in EUV lithography. Such reflective masks have a multilayer reflective film that reflects exposure light formed on a substrate, and an absorber film formed in a pattern on the multilayer reflective film that absorbs exposure light.
The reflective mask is manufactured from a substrate, a multilayer reflective film formed on the substrate, and a reflective mask blank having an absorber film formed on the multilayer reflective film, by forming an absorber film pattern by photolithography and the like.
As described above, due to the growing demand for miniaturization in the lithography process, significant problems are being encountered in the lithography process. One problem relates to defect information of mask blank substrates, substrates with multilayer reflective films and reflective mask blanks and the like used in the lithography process.
Mask blank substrates are being required to have even higher smoothness from the viewpoints of improving defect quality accompanying the miniaturization of patterns in recent years and the optical properties required of transfer masks.
In addition, substrates with multilayer reflective films are also being required to have even higher smoothness from the viewpoints of improving defect quality accompanying the miniaturization of patterns in recent years and the optical properties required of transfer masks. Multilayer reflective films are formed by alternately laminating layers that have a high refractive index with layers that have a low refractive index on the surface of a mask blank substrate. Each layer is typically formed by sputtering using sputtering targets composed of the materials that form these layers.
Ion beam sputtering is preferably used as the sputtering method from the viewpoint of not requiring the generation of plasma by electrical discharge and being resistant to contamination by impurities present in the multilayer reflective film, and from the viewpoint of having an independent ion source thereby making setting of conditions comparatively easy. In addition, from the viewpoint of the smoothness and surface uniformity of each layer formed, the high refractive index layer and low refractive index layer are deposited by allowing sputtered particles to reach the target at a large angle with respect to the normal (line perpendicular to the main surface of the mask blank substrate) of a main surface of the mask blank substrate, or in other words, at an angle diagonal or nearly parallel to a main surface of the substrate.
Patent Literature 1 describes a technology for manufacturing a substrate with a multilayer reflective film using such a method in which, when depositing a multilayer reflective film of a reflective mask blank for EUV lithography on a substrate, ion beam sputtering is carried out by maintaining the absolute value of an angle α formed between the normal of the substrate and sputtered particles entering the substrate such that 35 degrees≦α≦80 degrees while rotating the substrate about the central axis thereof.
In addition, Patent Literature 2 describes a reflective mask blank for EUV lithography in which an absorber layer that absorbs EUV light contains Ta, B, Si and N, the content of B is not less than 1 at % to less than 5 at %, the content of Si is 1 at % to 25 at %, and the composition ratio between Ta and N (Ta:N) is 8:1 to 1:1.